It's so easy to stay fit in the summer -- the beautiful days keep you outdoors and active, while the heat controls your appetite. Then the weather turns, and every day it seems you're moving just a little bit less and eating just a little bit more.

But when you think about it, there's really no excuse for such behavior. "It's almost like a woman who's pregnant and thinks, â€˜I can eat anything I want!'" says Joshua Margolis, founder of New York Cityâ€“based personal training service Mind Over Matter Health & Fitness. "You can, but it's not necessarily the right thing to do." In other words, winter is not a license to sit around and stuff your face. If it were, Colorado would be a state full of fat people; instead, it has boasted the nation's lowest percentage of obese adults since 1990, a fact largely attributed to the population's strong outdoor culture.

However, even if we accept that blaming the cold for our sloth-like habits is wrong, it can be tough to stay on track with health goals when it's cozy inside and miserable outside. To help, we asked fitness expert Margolis and nutrition expert Ilyse Schapiro, a registered dietitian and certified dietitian/nutritionist at Brown & Medina Nutrition in New York City, for their best tips on how to stay healthy and happy during those long months of sleet and snow.

Eat Citrus

"Winter is cold and flu season, so it's more important than ever to keep your immune system functioning at its best," says Schapiro, who recommends taking vitamin C to give it that extra boost. "Oranges and clementines are in season, so why not put out a bowl of those during the game instead of a bowl of chips?"

Find a Workout Partner

It's a dark, bitter morning, and your bed's so warm and comfy ... but you're supposed to meet Joe at indoor tennis in 30 minutes. Says Margolis: "Having a buddy to exercise with automatically creates accountability and responsibility." If you're in need of a partner, ask that guy you always see at the gym or someone from your intramural sport team, or simply post on Facebook: "Hey, I'm going out for a run in the morning. Anybody interested in joining?"

Choose Healthy Comfort Foods

We crave thick, heavy food in the winter. But instead of reaching for a can of calorie-packed cream-based soup, heat up some hearty lentil stew or veggie chili. Schapiro also recommends preparing recipes with low-fat instead of full-fat dairy and lean ground turkey versus ground beef. As for carbs, always opt for whole grains, including whole-wheat pasta, whole-wheat couscous, quinoa and brown rice. For more ideas, check out EatingWell, Schapiro's go-to source for healthy recipes.

Buy Winter Workout Gear

When you make a financial commitment to something, you tend to stick to it more. Likewise, spend money on a parka, goggles, base layers, ice tools -- the sky's the limit -- and you're probably not going to let them go to waste. For an extra guarantee, put your purchases somewhere you'll see them every time you come in or out the door. Guilt is a brilliant motivator.

Get Your Beta-carotene

Antioxidants protect against damage to cells and can help fight diseases and illnesses from cancer to the common cold. If you increase your intake in the winter, says Schapiro, you can stave off or shorten the length of a cold. Beta-carotene is one major antioxidant, and foods rich in it are readily available during winter. Carrots, sweet potatoes and broccoli are all in season and are packed with the infection-fighting cells.

Pick up a Winter Sport

Skiing, snowboarding, snowshoeing, ice hockey, ice climbing, snowmobiling â€¦ "There are an inordinate amount of fitness disciplines that require colder temperatures," says Margolis. The winter season is a great reminder to mix up your workout. "If you're doing the same thing repeatedly," explains Margolis, "your body gradually gets used to it, and the energy you expend decreases. It's no longer as challenging for your body." Ramp up your workout by testing new cold-weather skills. Or simply get out there with your little cousins and have a snowball fight; 8-year-old kids can make you burn more calories than drill sergeants.

Caroline Kinneberg is a Paris-based journalist who has written for The New York Times, The Boston Globe and Vice.